Imagine a huge salt flat like Bonneville or Salar de Uyuni. Due to other logistical concerns, commerce between the economic powers on either side of the flat is forced travel across it. This has resulted in the regular caravan crossing this flat slowly growing to the size of an city.

Typically, the city remains still on either end of the flat. When the rainy season arrives the flat floods with an inch or two of water and the predictable seasonal winds pick up. The city then raises its sails, and like the sailing stones of Racetrack Playa, it rides the wind to the other side.

How feasible is this setup? What city construction traits would be required? What natural conditions would encourage this?

A few notes on the situation:

  • The city is built atop numerous parallel skids that are the only places it touches the ground.

  • It's built of the lightweight materials like balsa wood and cloth. People pay handsomely for passage in the city and for every ounce of cargo they bring.

  • The salt flat environment is somewhat hostile to most animals. Beasts of burden, like horses or oxen, have a ~50% survival rate when attempting to cross without the city.

  • The city doesn't need to be a single cohesive unit, but I'd like it to have a single central mobile structure instead of being a collection of tents and the like.

  • Age of sail era technology is available, but still rare and expensive in this region.

  • $\begingroup$ How the skids are constructed? No ball-bearings, by any chance? $\endgroup$ – Alexander Feb 14 '20 at 23:46
  • $\begingroup$ Ideally no ball bearings. Probably no wheels either if they can be avoided. The skids are probably more like sleds than anything else. $\endgroup$ – tinydoctor Feb 15 '20 at 2:03
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    $\begingroup$ It's going to be much tougher than sea barges, because of Stiction. $\endgroup$ – Alexander Feb 15 '20 at 5:40
  • $\begingroup$ It might be worth considering an ice flat rather than a salt flat. When ice is pressurised it can create a liquid film that reduces friction as happens with skates. When salt is compressed you just get stiction. $\endgroup$ – Slarty Feb 15 '20 at 13:51
  • $\begingroup$ It would be completely silly to build something the size of a city, when your commerce could easily take place via landsailers all during the year. Not to mention that unless you have winds that completely reverse direction seasonally, your city only makes one trip :-) $\endgroup$ – jamesqf Feb 16 '20 at 18:38

It would look pretty much like a clipper on steroids. To overcome drag and develop reasonable speed, an amount of surface area on the sails was needed similar if not larger than the size of the ship itself. This is in water which develops much less friction than land. To keep up speed and overcome friction an approximation of probably three times the sails compared to the size of your city would be needed, if not more. I would imagine large constructs on the outside of your city would exists, purely for extra sails. which are unusable for habitation or storage.

Apart from the sails two biggest problems you would face is friction and manpower. To operate a clipper sized ship, historically a crew of around 250 people was used. (this was mostly done to offset the mortality rate on board, due to disease, effectively a 100 man crew could suffice). Still a proportionally large crew would be needed for your city, likely much larger than what could permanently live on board. A smaller crew could be employed if manoeuvrability isn't an issue, depending on how precise your directional winds are. But it is unlikely you get a wind going precisely where you need, constant enough for the entire trip, so adjustments to the sails would constantly be needed. They would need to work in shifts as well, to keep a constant speed 24/7, because stopping would be extremely problematic.

The friction, or stiction as mentioned in the comments, would be fairly enormous due to the size and weight of your city. Even when built from lightweight materials, a reasonable strength and structural integrity would be needed, so weight would have to be added to achieve this. To achieve enough force to get the city moving, most likely an external push would be needed initially. Especially so if the city has been static for a season and the sleds rained into the ground. Even then the sleds would possibly leave large grooves in the ground due to the weight, and you would be more likely to effectively sail through the ground than over it.

  • $\begingroup$ This is helpful. I was considering the friction problem, but hadn't thought of the manpower problem. The initial push requirement adds an interesting element too. $\endgroup$ – tinydoctor Feb 15 '20 at 17:41
  • $\begingroup$ "To operate a clipper sized ship, often a crew of around 400 people was used." - Hmm... Cutty Sark, not a small clipper ship, had a complement of only 28-35. $\endgroup$ – Alexander Feb 16 '20 at 7:59
  • $\begingroup$ @Alexander it has been a while since I have been to a maritime museum, so I might not remember the best. But I have looked into VOC ships, which are fairly the same size as clippers, which mostly set off with a crew of around 250 men. Granted, this is not as high, and was only done to offset the mortality rate on board. An efficient crew of 100 would be technically possible with modern hygiene and healthcare. I will adjust my answer. $\endgroup$ – Plutian Feb 16 '20 at 8:32
  • $\begingroup$ A crew number much depends on duties. 17-18 century merchant ships had to defend themselves and had a number of guns on board, which effectively required to double the complement. My 19-century example had just an essential sailing crew. $\endgroup$ – Alexander Feb 16 '20 at 10:12
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    $\begingroup$ To overcome friction, you might consider the overcraft principle. If the ground is very flat, build your city on planks and inject air below the planks from a hole in the middle... $\endgroup$ – dargaud May 28 '20 at 13:40

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